The present invention relates to an opto-electronic scale reading apparatus and scale therefor, forming two-direction measurement apparatus.
A known form of opto-electronic scale reading apparatus for measuring relative displacement of two members comprises a scale on one of the members, having scale marks forming a periodic pattern, a read head provided on the other member, means for illuminating the scale, periodic diffraction means for interacting with light from the scale marks to produce interference fringes having movement relative to the read head and detecting means in the read head responsive to the fringes to produce a measure of the displacement.
An example of such apparatus is disclosed in EP-A-0 207 121 and also U.S. Pat. No. 4,974,962, each of which shows the means for illuminating and the periodic diffraction means in the read head. U.S. Pat. No. 4,926,566 discloses a method of producing a scale, in the form of a flexible tape produced by rolling, the pitch of the scale marks being 20 xcexcm or 40 xcexcm for example. The illuminating means, the diffraction means and the detecting means responsive to the fringes may be integrated in the read head in the manner described in U.S. Pat. No. 5,302,820.
However, in the above basically only displacement in one linear direction may be measured. One way to measure displacement in two orthogonal linear directions is to use two read heads in one body at right angles to one another. However, measurement errors, particularly Abbxc3xa9 errors, can result from the necessary spacing between the two read heads. Such an arrangement may also be expensive or not compact.
Another known form of opto-electronic scale reading apparatus is disclosed in U.S. Pat. No. 5,204,524. The scale comprises a diffraction grating co-operating with at least one other grating on a read head to produce interference fringes, some or all of which move relative to the read head during a displacement of the read head relative to the scale and the measurement is a count of such fringes. Such a scale has to be of diffraction quality, and the accuracy and reliability of the measurement depends on such parameters as the regularity of the spacing of the scale marks, the sharp definition of the edges of the marks and the freedom of the scale from scratches and like imperfections. Such a scale can be expensive to produce and protect especially when the scale has to be relatively large.
It is also known to increase the number of signals obtainable from any two adjacent marks of the scale by phase quadrature interpolation and known scale-reading apparatus can be subject to phase errors.
In each of U.S. Pat. No. 5,204,524 and U.S. Pat. No. 5,576,537 there is shown apparatus for measuring displacement in each of two directions between two members wherein use is made of a scale capable of producing diffracted orders. Such a scale must necessarily have marks which have a single periodicity in each of the two directions and there is no indication of the presence of secondary periodicities defining departures from a single periodicity in each of the two directions, or of a spatial filter having a pass band defining a maximum such departure.
According to the present invention, there is provided measurement apparatus comprising a scale extending in two directions and scale reader apparatus for determining displacement during relative movement between the scale and the scale reader apparatus in each of the two directions, the scale comprising a matrix pattern having periodicity in the two directions, the scale reader apparatus comprising a scale illuminator, a diffraction grating structure and a light detector, in use the illuminator acting to illuminate the scale, the grating structure causing light from the scale to be formed into an interference pattern having light of varying intensity and the detector acting to detect the interference pattern and produce an indication of the displacement in each of the two directions, wherein the grating structure provides a grating in one of the two directions interlaced within a grating in the other of the two directions.